A vehicle powered primarily by an engine includes a battery serving as a power source for a starter motor used to start the engine. A typical example of such a battery is a lead-acid battery. In recent years, charge-discharge characteristics of a lead-acid battery have improved. With this improvement, a lead-acid battery is increasingly common as a power source for a special electric vehicle, such as an electric cart and a fork lift, which conventionally uses a lithium-ion secondary battery so expensive as to make the special electric vehicle unprofitable.
A dead battery or a battery that has been degraded in performance ranks first in a number of troubles that private vehicles suffer (specifically, the number indicates how many times Japan Automobile Federation (JAF) is called to come to the rescue of vehicles). In recent years, a stop-start system has become more common in a vehicle powered primarily by an engine in an effort to reduce emissions. However, while the stop-start system stops the engine, a remaining capacity of a battery may decrease to a point where the battery cannot generate an output high enough to restart the engine. Accordingly, it is desirable to detect a remaining capacity of a battery with high accuracy so that such a battery problem is prevented (see, for example, PTL 1).
Generally, an open circuit voltage (hereinafter referred to as “OCV”) and a remaining capacity of a lead-acid battery are known to be linearly related. PTL 1 describes a technique of calculating a remaining capacity, based on an OCV measured, using the linear relationship.
PTL 2 discloses an invention that accurately estimates a state of charge (hereinafter referred to as “SOC”), which is a remaining capacity of a battery, by constructing an equivalent circuit model of the battery in consideration of a polarization component and estimating an internal state of the battery with high accuracy.